Steel sheet having excellent wear resistance and composite corrosion resistance, and method of manufacturing same

ABSTRACT

The present invention provides a steel sheet having excellent wear resistance and composite corrosion resistance, and a method of manufacturing same. According to one example of the present invention, a corrosion-resistant steel sheet having excellent wear resistance and composite corrosion resistance comprises, in wt %: 0.04-0.10% of carbon (C); 0.10% or less (excluding 0%) of silicon (Si); 0.20-0.35% of copper (Cu); 0.1-0.2% of nickel (Ni); 0.05-0.15% of antimony (Sb); 0.07-0.22% of tin (Sn); 0.05-0.15% of titanium (Ti); 0.01% or less (excluding 0%) of sulfur (S); 0.005% or less (excluding 0%) of nitrogen (N); 0.05-0.15% of molybdenum (Mo); and the balance of iron (Fe) and inevitable impurities, and satisfies formula 1 and formula 2 below.[Ni]/[Cu]≥0.5   [Formula 1]48×([Ti]/48−[S]/32−[N]/14)≥0.04   [Formula 2]Here, in formula 1 and formula 2, [Ni], [Cu], [Ti], [S], and [N] represent Ni, Cu, Ti, S, and N content (wt %) in the steel sheet, respectively.

TECHNICAL FIELD

The present invention relates to a steel sheet having excellent wear resistance and composite corrosion resistance and a method of manufacturing the same. More particularly, the present invention relates to a steel sheet having corrosion resistance to a phenomenon in which a steel sheet corrodes, due to sulfate acid/hydrochloric acid composite condensate water and sulfuric acid condensate water produced by SO_(x), Cl, and the like present in exhaust gas after fossil fuel combustion when an exhaust gas temperature is lowered, high strength, and excellent wear resistance, and a method of manufacturing the same.

BACKGROUND ART

Fossil fuel includes various impurity elements such as S and Cl. Since combustion is performed using the fossil fuel, there is always a problem in that piping which is a passage through which combustion gas passes and equipment are deteriorated by corrosion. The corrosion phenomenon is referred to as condensate water corrosion, and representative uses where piping and equipment are exposed to the corrosion environment are exhaust gas piping and environmental facilities of thermal power plants, automobile exhaust system, and the like. The kinds of condensate corrosion includes sulfuric acid condensate water corrosion in which S included in exhaust gas burns to form SO_(x), and in particular, SO₃ meets moisture in the exhaust gas to form sulfuric acid, corrosion by hydrochloric acid condensate water in which chlorine included in exhaust gas or industrial water produces hydrochloric acid by various reactions, sulfuric acid/hydrochloric acid composite condensate water corrosion occurring when the sulfuric acid and the hydrochloric acid are mixed in combination, and the like. A starting temperature of the acid condensation is related to the temperature of the exhaust gas itself, contents of SO_(x) and Cl in the exhaust gas, and a water vapor content.

In recent years, for the purpose of utilizing power generation efficiency or waste heat discharged to the outside in uses such as power plants, a demand to lower the exhaust gas temperature itself continues. In general, when the exhaust temperature is lowered to the temperature at which sulfuric acid starts to condense, sulfuric acid gas formed in the exhaust gas is liquefied and condensed on the surface of steel material to increase an amount to cause corrosion, and when the exhaust gas temperature is lowered to a lower temperature at which hydrochloric acid may be condensed, a composite corrosion phenomenon in which sulfuric acid and hydrochloric acid are condensed in combination occurs.

In addition, a study related to facility change for increasing desulfurization efficiency in thermal power plant environmental equipment continues. As a representative example, Gas Gas Heater (GGH) type which is a heat exchange device at a front/rear end of the desulfurization equipment is being changed. Conventional GGH is disposed at a rear end of an electrostatic precipitator (EP), and development of a steel material used herein was studied with a focus on corrosion resistance, but recently, in GGH, since a part of desulfurization equipment was disposed at a front end of the electrostatic precipitator, corrosion by steel material erosion by dusts which have not been removed, and also, corrosion from wear occur, and thus, for the steel material used in the equipment, a wear resistance problem should be solved in addition to a corrosion resistance problem.

As an example of a solution to the problem, there are a method of using a high alloy-based high corrosion-resistant steel such as Duplex-based stainless steel (STS) or a method of raising an exhaust gas temperature, but the methods cause high costs and a decrease in power generation efficiency. In addition, there was a movement to adopt a high-strength steel material, and this solves a strength problem, but may cause a problem of deterioration of other facilities due to the corrosion resistance problem.

Meanwhile, when a Cu-added corrosion-resistant steel known as a sulfuric acid-resistant condensed corrosion steel is used, a Cu concentrated layer produced on the surface of steel exhibits corrosion resistance to sulfuric acid condensation to form a corrosion suppression layer to suppress corrosion, and an effect of greatly improving an equipment life is exhibited, for the preparation of the case of using a general steel. However, a lower temperature of the exhaust gas, complexity of the corrosion environment, and requirement of wear resistance, as mentioned above lower the corrosion resistance characteristics to sulfuric acid-resistant condensed corrosion steel, and thus, there has been a demand for a corrosion-resistant steel having better performance.

Further, there has been a problem in that its original performance was not exhibited under complex and harsh corrosion-resistant environments with a conventional sulfuric acid-resistant condensed corrosion steel or a high-alloy stainless steel.

DISCLOSURE Technical Problem

The present invention has been made in an effort to provide a steel sheet having excellent wear resistance and composite corrosion resistance, and a method of manufacturing the same.

More specifically, a steel sheet having corrosion resistance to a phenomenon in which a steel sheet corrodes, due to sulfate acid/hydrochloric acid composite condensate water and sulfuric acid condensate water produced by SO_(x), Cl, and the like present in exhaust gas after fossil fuel combustion when an exhaust gas temperature is lowered, high strength, and excellent wear resistance, and a method of manufacturing the same are provided.

Technical Solution

An exemplary embodiment of the present invention provides a corrosion-resistant steel sheet having excellent wear resistance and composite corrosion resistance including, by weight: 0.04 to 0.10% of carbon (C), 0.10% or less (excluding 0%) of silicon (Si), 0.20 to 0.35% of copper (Cu), 0.1 to 0.2% of nickel (Ni), 0.05 to 0.15% of antimony (Sb), 0.07 to 0.22% of tin (Sn), 0.05 to 0.15% of titanium (Ti), 0.01% or less (excluding 0%) of sulfur (S), 0.005% or less (excluding 0%) of nitrogen (N), and 0.05 to 0.15% of molybdenum (Mo), with a balance of iron (Fe) and inevitable impurities, the corrosion-resistant steel sheet satisfying the following Formulae 1 and 2:

[Ni]/[Cu]≥0.5   [Formula 1]

48×([Ti]/48−[S]/32−[N]/14)≥0.04   [Formula 2]

wherein [Ni], [Cu], [Ti], [S], and [N] represent contents (wt %) of Ni, Cu, Ti, S, and N in the steel sheet, respectively.

The corrosion-resistant steel sheet according to an exemplary embodiment of the present invention includes a TiC precipitate, and the TiC precipitate and an aggregate formed of the TiC precipitate may be included at 10¹⁶ or more per 1 cm³.

The TiC precipitate may have a particle diameter of 1 to 10 nm.

The corrosion-resistant steel sheet according to an exemplary embodiment of the present invention may satisfy the following Formula 3:

10×[Mo]+12×[Sn]+22×[Sb]+50×[Cu]≥16   [Formula 3]

wherein [Mo], [Sn], [Sb], and [Cu] represent contents (wt %) of Mo, Sn, Sb, and Cu in the steel sheet, respectively.

When the steel sheet is immersed in a mixed solution of 28.5 wt % of a sulfuric acid solution and 0.5 wt % of a hydrochloric acid solution at 40 to 80° C., a concentrated layer may be formed on the surface of the steel sheet.

When the steel sheet is immersed in 50 wt % of a sulfuric acid solution at 50 to 90° C., a concentrated layer may be produced on the surface of the steel sheet.

The concentrated layer may include Mo, Cu, Sb, and Sn.

The concentrated layer may have a concentrated amount of 10 wt % or more.

Here, the concentrated amount refers to the sum of the contents (wt %) of concentrating elements Mo, Cu, Sb, and Sn, at a boundary point at which Fe and O have the same wt %.

The concentrated layer may have a thickness of 10 nm or more.

A recrystallization fraction after subjecting the steel sheet to an annealing heat treatment may be 80% or more.

When the steel sheet is immersed in a mixed solution of 28.5 wt % of a sulfuric acid solution and 0.5 wt % of a hydrochloric acid solution at 60° C. for 6 hours, a corrosion weight loss ratio may be 8.0 mg/cm²/hr or less.

When the steel sheet is immersed in 50 wt % of a sulfuric acid solution at 70° C. for 6 hours, a corrosion weight loss ratio may be 37 mg/cm²/hr or less.

The product of the corrosion weight loss ratio when the steel sheet is immersed in 50 wt % of the sulfuric acid solution at 70° C. for 6 hours and the corrosion weight loss ratio when the steel sheet is immersed in the mixed solution of 28.5 wt % of the sulfuric acid solution and 0.5 wt % of the hydrochloric acid solution at 60° C. for 6 hours may be 20 or less.

When the steel sheet is a hot rolled steel sheet, the hot rolled steel sheet may have a tensile strength of 580 MPa or more and a surface hardness of 85 or more based on HRB.

When the steel sheet is a cold rolled steel sheet, the cold rolled steel sheet may have a tensile strength of 540 MPa or more and a surface hardness of 80 or more based on HRB.

Another embodiment of the present invention provides a method of manufacturing a corrosion-resistant steel sheet including: preparing a steel slab which includes, by weight: 0.04 to 0.10% of carbon (C), 0.10% or less (excluding 0%) of silicon (Si), 0.20 to 0.35% of copper (Cu), 0.1 to 0.2% of nickel (Ni), 0.05 to 0.15% of antimony (Sb), 0.07 to 0.22% of tin (Sn), 0.05 to 0.15% of titanium (Ti), 0.01% or less (excluding 0%) of sulfur (S), 0.005% or less (excluding 0%) of nitrogen (N), and 0.05 to 0.15% of molybdenum (Mo), with a balance of iron (Fe) and inevitable impurities, and satisfies the following Formulae 1 and 2; heating the slab at 1,200° C. or higher; and hot rolling the heated slab at a finish rolling temperature of 850 to 1000° C. to manufacture a hot rolled steel sheet:

[Ni]/[Cu]≥0.5   [Formula 1]

48×([Ti]/48−[S]/32−[N]/14)≥0.04   [Formula 2]

wherein [Ni], [Cu], [Ti], [S], and [N] represent contents (wt %) of Ni, Cu, Ti, S, and N in the steel sheet, respectively.

After the step of manufacturing the hot rolled steel sheet, winding the hot rolled steel sheet at 450 to 750° C.; cold rolling the wound hot rolled steel sheet at a reduction rate of 54 to 70% to manufacture a cold rolled steel sheet; and subjecting the cold rolled steel sheet to an annealing heat treatment at 750 to 880° C., may be further included.

In the step of heating the slab at 1,200° C. or higher, a residence time may be 150 minutes or more.

Advantageous Effects

The corrosion-resistant steel sheet according to an exemplary embodiment of the present invention may be useful for piping through which exhaust gas passes after combustion of fossil fuel, hot rolled products for fossil fuel combustion equipment, and raw materials of cold rolled products.

When the corrosion-resistant steel sheet according to an exemplary embodiment of the present invention is applied to Gas Gas heater (GGH) equipment, both wear resistance and composite corrosion resistance requirements may be met in spite of a large difference in environmental changes, regardless of whether the GGH equipment which is a heat exchange device used in a desulfurization equipment for thermal power plants is installed at a front end or a rear end of an electrostatic precipitator (EP).

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing an element concentration degree of a surface portion of a steel sheet, by measuring an element distribution from the surface to the inside by GDS measurement, after the steel sheet of Inventive Example 2 was immersed in 50 wt % of a sulfuric acid solution for 24 hours.

FIG. 2 shows (a) a microstructure when Inventive Example 4 was wound at a hot rolling winding temperature of Condition 1, and (b) a microstructure when the component system of Inventive Example 4 was wound at a low winding temperature of Condition 4.

MODE FOR INVENTION

In the present specification, the terms such as first, second, and third are used for describing various parts, components, areas, layers, and/or sections, but are not limited thereto. These terms are used only for distinguishing one part, component, area, layer, or section from other parts, components, areas, layers, or sections. Therefore, a first part, component, area, layer, or section described below may be mentioned as a second part, component, area, layer, or section without departing from the scope of the present invention.

In the present specification, when it is said that a portion “comprises” a constituent element, it means that other constituent elements may be further comprised rather than other constituent elements are excluded, unless otherwise stated to the contrary. In the present specification, the terminology used herein is only for mentioning a certain example, and is not intended to limit the present invention. Singular forms used herein also include plural forms unless otherwise stated clearly to the contrary. The meaning of “comprising” used in the specification is embodying certain characteristics, regions, integers, steps, operations, and/or components, but is not excluding the presence or addition of other characteristics, regions, integers, steps, operations, and/or components.

In the present specification, the term “combination thereof” included in a Markush-type expression refers to a mixture or a combination of one or more selected from the group consisting of constituent elements described in a Markush-type expression, and refers to inclusion of one or more selected from the group consisting of the constituent elements.

In the present specification, when it is mentioned that a part is present “on” the other part, the part may be present directly on the other part, or another part may be involved between them. In contrast, when it is mentioned that a part is present “directly on” the other part, there is no part interposed between them.

Though not defined otherwise, all terms including technical terms and scientific terms used herein have the same meaning as commonly understood by a person with ordinary skill in the art to which the present invention pertain.

Terms defined in commonly used dictionaries are further interpreted as having a meaning consistent with the related technical literatures and the currently disclosed description, and unless otherwise defined, they are not interpreted as having an ideal or very formal meaning.

In addition, unless otherwise particularly stated, % means wt %, and 1 ppm is 0.0001 wt %.

The meaning of further including an additional element in an exemplary embodiment of the present invention is including an additional element in place of iron (Fe) as a balance at the added amount of the additional element.

Hereinafter, an exemplary embodiment of the present invention will be described in detail so that a person with ordinary skill in the art to which the present invention pertains may easily practice the invention. However, the present invention may be implemented in various forms, and is not limited to the exemplary embodiments described herein.

The inventors of the present invention confirmed that when an element to form precipitates such as Ti is added to a common medium-low carbon steel sheet, in the case of using appropriate manufacturing conditions in the manufacturing process, the hardness and the strength of a hot rolled material which is an intermediate material and a cold rolled material which is a final material may be greatly increased.

That is, the inventors of the present invention confirmed that when the steel sheet is subjected to a sulfuric acid or a sulfuric acid/hydrochloric acid composite corrosive environment, precipitates are formed by a corrosive product produced depending on the kind and the contents of the element contained in the steel sheet, and a composite relation, but additional corrosion is inhibited.

Here, when two or more of Mo, Cu, Sb, Sn, and the like which are special component element are added in combination to the steel sheet, corrosion resistance both in high-concentration sulfuric acid and sulfuric acid/hydrochloric acid component condensation environments may be greatly improved, and thus, it was concluded that equipment corrosion resistance performance in a condensate water corrosion environment may be significantly increased.

When a corrosion reaction is performed on a low-carbon steel sheet using the principle, it was confirmed that a concentrated layer containing corrosion-resistant elements produced between a steel material and a corrosion product may be densely formed, and thus, it was found that the steel sheet manufactured therefrom has excellent corrosion resistance in an immersion corrosive environment.

Hereinafter, as an exemplary embodiment of the present invention, a steel sheet having excellent wear resistance and composite corrosion resistance, and a method of manufacturing the same will be described in detail.

A corrosion-resistant steel sheet according to an exemplary embodiment of the present invention includes, by weight: 0.04 to 0.10% of carbon (C), 0.10% or less (excluding 0%) of silicon (Si), 0.20 to 0.35% of copper (Cu), 0.1 to 0.2% of nickel (Ni), 0.05 to 0.15% of antimony (Sb), 0.07 to 0.22% of tin (Sn), 0.05 to 0.15% of titanium (Ti), 0.01% or less (excluding 0%) of sulfur (S), 0.005% or less (excluding 0%) of nitrogen (N), and 0.05 to 0.15% of molybdenum (Mo), with a balance of iron (Fe) and inevitable impurities, and satisfies the following Formulae 1 and 2:

[Ni]/[Cu]≥0.5   [Formula 1]

48×([Ti]/48−[S]/32−[N]/14)≥0.04   [Formula 2]

wherein [Ni], [Cu], [Ti], [S], and [N] represent contents (wt %) of Ni, Cu, Ti, S, and N in the steel sheet, respectively.

Meanwhile, the corrosion-resistant steel sheet may further satisfy the following Formula 3:

10×[Mo]+12×[Sn]+22×[Sb]+50×[Cu]≥16   [Formula 3]

wherein [Mo], [Sn], [Sb], and [Cu] represent contents (wt %) of Mo, Sn, Sb, and Cu in the steel sheet, respectively.

First, the reasons for limiting the components of the steel sheet, and Formulae 1, 2, and 3 will be described.

Carbon (C): 0.04 to 0.10 wt %

A carbon content in a low-carbon steel sheet may be 0.04 to 0.10 wt %. When a carbon content in a steel is too high, deterioration of corrosion resistance, in particular, deterioration of sulfuric acid/hydrochloric acid composite corrosion resistance may occur by excessive TiC formation and carbide formation. On the contrary, when the carbon content is too low, it may be impossible to secure the strength to be desired in the present invention. More specifically, it may be 0.042 to 0.10 wt %.

Silicon (Si): 0.10 wt % or less (excluding 0 wt %)

A silicon content in the low-carbon steel sheet may be 0.10 wt % or less. When the silicon content in a steel is too high, a large amount of red scale may be caused by a composite phase shape of SiO₂ and Fe oxides on the surface. Therefore, the Si content may be in the range above for overcoming the surface defects. More specifically, it may be 0.05 wt % or less. More specifically, it may be 0.045 wt % or less. More specifically, it may be 0.01 to 0.045 wt %.

Copper (Cu): 0.20 to 0.35 wt %

Cu is a representative element which is, in the case of corrosion in an acid immersion environment, concentrated between the surface of a steel material and a corrosion product to prevent further corrosion. In order to show the effect, an appropriate amount of Cu may be added. However, when added too much, cracks may be caused in the manufacture due to the low melting point of Cu.

Nickel(Ni): 0.1% to 0.2 wt %

When only Cu is added to a steel without Ni, liquid Cu may penetrate to a grain boundary due to the low melting point of Cu to cause cracks. Ni is added for the purpose of raising the melting point with the addition to limit occurrence of cracks. When a Ni content is too low, it does not sufficiently serve to raise the melting point of Cu, and on the contrary, when the Ni content is too high, surface defects due to Ni may occur. More specifically, the Ni content may be 0.11 to 0.19 wt %.

[Ni]/[Cu]≥0.5   [Formula 1]

For the same reason as the reason for adding Ni with Cu, Ni and Cu may be added in the above range, in order to appropriately increase a melting point and not to cause surface defects due to Ni. When the value of Formula 1 is too high, surface defects due to Ni may occur, and when the value of Formula 1 is too low, an effect of raising a melting point by Ni may be insignificant. In Formula 1, [Ni] and [Cu] represent contents (wt %) of Ni and Cu in the steel sheet, respectively.

Antimony (Sb): 0.05 to 0.15 wt %

Sb is added for forming a stable concentrated layer on the surface, like Cu. When the Sb content is too low, a sufficient concentrated layer may not be formed. On the contrary, when the Sb content is too high, surface cracks may be caused.

Tin (Sn): 0.07 to 0.22 wt %

Sn is added for forming a stable concentrated layer on the surface like Cu and Sb. In particular, it was confirmed that Sn is first dissolved in an acid immersion environment such as sulfuric acid to serve to greatly improve steel type corrosion resistance. More specifically, though it is not clear, it is considered that Sn improves steel type corrosion resistance by the following mechanism. When the steel sheet is under an immersion environment of sulfuric acid or composite acid, Sn and Cu are dissolved, and Sn is dissolved before Cu. As Sn is dissolved before Cu, Sn is dissociated in the solution. The dissociated Sn lowers corrosion potential of the solution, thereby partially delaying a corrosion phenomenon of the steel sheet. Here, a corrosion potential refers to a potential to a combination electrode (reference electrode) of metal undergoing corrosion. In addition, a corrosion delay layer may be formed in a process in which dissolved Sn is fused again on the surface of the steel sheet, and it is considered that the corrosion delay layer may delay corrosion of the steel sheet. When Sn is included too little, a sufficient concentrated layer may not be formed. When Sn is added too much, serious surface cracks may be caused in the production process. More specifically, the Sn content may be 0.073 to 0.22 wt %.

Molybdenum (Mo): 0.05 to 0.15 wt %

Mo is added for forming a stable concentrated layer on the surface, like Sn, Sb, Cu, and the like. In particular, it was confirmed that Mo shows particularly excellent corrosion resistance under an environment where a sulfuric acid and a hydrochloric acid are condensed in combination as well as a sulfuric acid condensation environment. When the content of Mo is too low, sufficient corrosion resistance to complex corrosion resistance is not secured, and this is considered that Mo does not sufficiently serves to prevent continuous release of the concentrated layer by the intervention of Cl. However, since Mo is an element which significantly increases hardenability, when added too much, a martensite or bainite phase may be formed in the steel, which causes reduced corrosion resistance. In addition, in an annealing process after cold rolling, a recrystallization temperature is excessively increased to cause rapid reduction of ductility after cold rolling. More specifically, it may be 0.07 to 0.15 wt %.

10×[Mo]+12×[Sn]+22×[Sb]+50×[Cu]≥16   [Formula 3]

Mo, Cu, Sb, and Sn are elements which form a concentrated layer on the surface of the steel sheet under a sulfuric acid/hydrochloric acid composite condensation atmosphere or a sulfuric acid condensation atmosphere, and the relationship of Formula 3 as well as appropriate contents of each element may be satisfied. When the value of Formula 3 is too low, a sufficient concentrated layer may not be formed. In Formula 3, [Mo], [Sn], [Sb], and [Cu] represent contents (wt %) of Mo, Sn, Sb, and Cu in the steel sheet, respectively. More specifically, Formula 3 may be 16 to 27. More specifically, Formula 3 may be 16.3 to 21.86.

Titanium (Ti): 0.05 to 0.15 wt %

Ti acts as an element which forms precipitates and is added for increasing the strength and the wear resistance of the steel sheet. That is, Ti is bonded to C to form a TiC precipitate. TiC is a fine precipitate, may improve the hardness and the wear resistance of a steel sheet due to precipitation strengthening, and also, may increase strength. In this regard, the details of TiC will be described later. Here, a Ti content is too low, precipitates are not sufficiently formed to show no strength increase effect. However, when the Ti content is too high, TiC is excessively formed to cause cracks in rolling, and Ti and Al-based composite oxides are formed in a steelmaking step to block a tundish nozzle to cause manufacturing defects and surface defects. Therefore, more specifically, Ti may be included at 0.05 to 0.145 wt %. More specifically, Ti may be included at 0.052 to 0.145 wt %.

Sulfur (S): 0.01 wt % or less (excluding 0%)

S may cause a reverse effect of limiting a Ti content effective for forming a Ti carbide. The reason is that the present invention is characterized by increasing wear resistance by precipitation hardening by TiC precipitate formation, but since TiS is formed before TiC is formed, the more S content interferes with the formation of TiC. Therefore, the maximum component range may be the range described above. More specifically, it may be 0.0097 wt % or less. More specifically, it may be 0.001 to 0.0097 wt %.

Nitrogen (N): 0.005 wt % or less (excluding 0%)

N may cause a reverse effect of limiting a Ti content effective for forming a Ti carbide. The reason is that the present invention is characterized by increasing wear resistance by precipitation hardening by TiC precipitate formation, but since TiN is formed before TiC is formed, the more N content interferes with the formation of TiC. For reference, when Ti forms precipitates, they are formed in the order of TiN, TiS, and TiC. Therefore, the range of the maximum component may be in the above range. More specifically, it may be 0.004 wt % or less. More specifically, it may be 0.001 to 0.004 wt %.

48×([Ti]/48−[S]/32−[N]/14)≥0.04   [Formula 2]

The effective Ti(Ti*) content may be calculated by Formula 2. Even in the case in which the component ranges of S and N are satisfied, when the range of Formula 2 is not satisfied, sufficient TiC may not be formed to cause drop in strength. Here, in Formula 2, [Ti], [S], and [N] represent contents (wt %) of Ti, S, and N in the steel sheet, respectively. More specifically, the range of Formula 2 may be 0.04 to 0.12.

In addition, the steel sheet may further include manganese (Mn) and aluminum (Al).

Manganese (Mn): 0.5 to 1.5 wt %

Mn serves to improve strength by solid solution strengthening in a steel, but when the content is too excessive, coarse MnS is formed to rather deteriorate strength. Therefore, it is preferred that the Mn content in the present invention is limited to 0.5 to 1.5 wt %.

Aluminum(AI): 0.02 to 0.05 wt %

Al is an element which is inevitably added in the manufacture of an aluminum-killed steel, and it is preferred to add Al in an appropriate content for a deoxidation effect. However, the Al content is more than 0.02 wt %, surface defects of the steel sheet are more likely to be caused, and weldability may be reduced. Therefore, in the present invention, it is preferred to limit the Al content to 0.02 to 0.05 wt %.

In addition to the components, the present invention includes Fe and inevitable impurities. Since the inevitable impurities are well known in the art, detailed description thereof will be omitted. In an exemplary embodiment of the present invention, addition of effective components other than the above components is not excluded, and when an additional component is further included, it is included by replacing the balance of Fe.

Meanwhile, the corrosion-resistant steel sheet according to an exemplary embodiment of the present invention is characterized by having excellent wear resistance, and includes a TiC precipitate in this regard. The TiC precipitate is a fine precipitate and may improve the hardness and the wear resistance of a steel sheet due to precipitation hardening, and also, may increase strength.

The TiC precipitate and the aggregate formed of the TiC precipitate may be included at 10¹⁶ or more per 1 cm³. When the precipitate content is too low, the strength and the wear resistance to be desired may not be secured. More specifically, they may be included at 10¹⁶ to 10¹⁸ per 1 cm³.

The TiC precipitate may have a spherical shape.

The TiC precipitate may have a particle diameter of 1 to 10 nm. The precipitates interfere with movement of potential inside a steel material and form a potential band to increase strength, and when the particle diameter of the precipitates is too small, potential may easily move so that there is no effect of strength increase, but when the particle diameter of the precipitates is too large, potential cuts and passes through the precipitates to facilitate movement, and thus, the effect of strength increase is also deteriorated. More specifically, the particle diameter of the TiC precipitate may be 2 to 8 nm. Here, a particle diameter refers to a diameter of a sphere, when the sphere having the same volume as the particle is assumed.

In addition, the TiC precipitate may be uniformly distributed in the steel sheet.

Meanwhile, on the corrosion-resistant steel sheet according to an exemplary embodiment of the present invention, Cu, Sb, Sn, and the like form a concentrated layer under a sulfuric acid/hydrochloric acid composite condensate atmosphere or a sulfuric acid condensation atmosphere, which suppresses additional corrosion. More specifically, when the steel sheet is immersed in a mixed solution of 28.5 wt % of a sulfuric acid solution and 0.5 wt % of a hydrochloric acid solution at 40 to 80° C., the concentrated layer may be produced on the surface of the steel sheet. In addition, when the steel sheet is immersed in 50 wt % of a sulfuric acid solution at 50 to 90° C., the concentrated layer may be produced on the surface of the steel sheet. More specifically, when the steel sheet is immersed therein for 4 to 8 hours, the concentrated layer may be produced.

Here, the concentrated layer refers to a layer in which Mo, Cu, Sb, and Sn start to be concentrated, and on the other hand, it is generally similar to the point at which oxidation starts. The concentrated layer in the present invention refers to a layer in which the total amount of Mo, Cu, Sb, and Sn is more than 4 times the total amount of Mo, Cu, Sb, and Sn of the steel sheet.

In addition, the concentrated layer may be an amorphous concentrated layer.

The concentrated layer is produced with the formation of a corrosive layer when immersed in an acid. Here, the corrosive layer refers to a layer in which Fe is oxidized by O. In general, Fe is oxidized before Mo, Cu, and Sb, and when immersed in an acid, Fe is dissociated into a Fe ion and escapes into an acid solution, but Mo, Cu, and Sb are stable in a solid state and remain on the surface. Therefore, though the acid reaction continues and a Fe content reduction continuously occurs on the surface of the steel sheet, Mo, Cu, and Sb remain on the surface to form a high concentration layer. This is produced on the surface in the form of a concentrated layer after a certain reaction time, and the concentrated layer prevents a direct contact between an acid and interior iron to suppress further corrosion.

The concentrated layer may include Mo, Cu, Sb, and Sn, and the concentrated amount of the concentrated layer may be 10 wt % or more as a total amount of Mo, Cu, Sb, and Sn. Here, the concentrated amount refers to the sum of the contents (wt %) of concentrating element Mo, Cu, Sb, and Sn, at a boundary point at which Fe and O have the same wt %. When the concentrated amount is too small, the concentrated layer is not sufficiently formed to increase a corrosion weight loss ratio. More specifically, it may be 11 to 27 wt %. The concentrated layer may have a thickness of 10 nm or more. More specifically, the concentrated layer may be formed at a thickness of 10 to 500 nm. When the concentrated layer is too thin, it is difficult for the concentrated layer to serve to prevent corrosion as described above. When the concentrated layer is formed to be too thick, cracks occur inside the concentrated layer, so that an acid may penetrate along the cracks to cause corrosion. More specifically, the concentrated layer may be formed at a thickness of 12 to 100 nm.

The corrosion-resistant steel sheet according to an exemplary embodiment of the present invention may be a hot rolled steel sheet or a cold rolled steel sheet.

In the case of the hot rolled steel sheet, the steel sheet may have a thickness of 2.5 to 5.5 mm. More specifically, the thickness may be 3.5 to 5.5 mm.

In the case of the cold rolled steel sheet, the steel sheet may have a thickness of 1.0 to 2.5 mm. More specifically, the thickness may be 1.0 to 2.0 mm.

When the corrosion-resistant steel sheet according to an exemplary embodiment of the present invention is a cold rolled steel sheet, a recrystallization fraction after subjecting the steel sheet to an annealing heat treatment may be 80% or more. More specifically, the recrystallization fraction may be 100%. When the recrystallization fraction is too low, the strength is increased, but ductility is rapidly decreased, and thus, defects are formed in the customer processing. Here, the recrystallization fraction refers to an area of grains which is recrystallized based on the total steel sheet area.

When the corrosion-resistant steel sheet according to an exemplary embodiment of the present invention is immersed in a mixed solution of 28.5 wt % of a sulfuric acid solution and 0.5 wt % of a hydrochloric acid at 60° C. for 6 hours, a corrosion weight loss ratio may be 0.8 mg/cm²/hr or less.

When the corrosion-resistant steel sheet according to an exemplary embodiment of the present invention is immersed in 50 wt % of a sulfuric acid solution at 70° C. for 6 hours, the corrosion weight loss ratio may be 37 mg/cm²/hr or less.

The product of the corrosion weight loss ratio when the corrosion-resistant steel sheet according to an exemplary embodiment of the present invention is immersed in 50 wt % of the sulfuric acid solution at 70° C. for 6 hours and the corrosion weight loss ratio when the steel sheet is immersed in the mixed solution of 28.5 wt % of the sulfuric acid solution and 0.5 wt % of the hydrochloric acid solution at 60° C. for 6 hours may be 20 or less.

When the corrosion-resistant steel sheet according to an exemplary embodiment of the present invention is a hot rolled steel sheet, the hot rolled steel sheet may have a tensile strength of 580 MPa or more and a surface hardness of 85 or more based on HRB.

When the corrosion-resistant steel sheet according to an exemplary embodiment of the present invention is a cold rolled steel sheet, the cold rolled steel sheet may have a tensile strength of 540 MPa or more and a surface hardness of 80 or more based on HRB.

A method of manufacturing a corrosion-resistant steel sheet according to an exemplary embodiment of the present invention includes: preparing a steel slab which includes, by weight: 0.04 to 0.10% of carbon (C), 0.10% or less (excluding 0%) of silicon (Si), 0.20 to 0.35% of copper (Cu), 0.1 to 0.2% of nickel (Ni), 0.05 to 0.15% of antimony (Sb), 0.07 to 0.22% of tin (Sn), 0.05 to 0.15% of titanium (Ti), 0.01% or less (excluding 0%) of sulfur (S), 0.005% or less (excluding 0%) of nitrogen (N), and 0.05 to 0.15% of molybdenum (Mo), with a balance of iron (Fe) and inevitable impurities, and satisfies the following Formulae 1 and 2; heating the slab at 1,200° C. or higher; and hot rolling the heated slab at a finish rolling temperature of 850 to 1000° C. to manufacture a hot rolled steel sheet:

[Ni]/[Cu]≥0.5   [Formula 1]

48×([Ti]/48−[S]/32−[N]/14)≥0.04   [Formula 2]

wherein [Ni], [Cu], [Ti], [S], and [N] represent contents (wt %) of Ni, Cu, Ti, S, and N in the steel sheet, respectively.

In addition, after the step of manufacturing the hot rolled steel sheet, winding the hot rolled steel sheet at 450 to 750° C.; cold rolling the wound hot rolled steel sheet at a reduction rate of 54 to 70% to manufacture a cold rolled steel sheet; and subjecting the cold rolled steel sheet to an annealing heat treatment at 750 to 880° C., may be further included.

Hereinafter, each step will be described in detail.

First, a slab satisfying the composition described above is heated. Since the reason for limiting the addition ratio of each composition in the slab is the same as the reason for limiting the composition of the steel sheet described above, repeated description will be omitted. Since the composition of the slab is substantially not changed in the manufacturing process such as hot rolling, winding, pickling, cold rolling, and annealing, as described later, the composition of the slab and the composition of the finally manufactured corrosion-resistant steel sheet is substantially the same.

By heating the slab, a subsequent hot rolling process is performed well, and the slab may be homogenized. More specifically, the heating may refer to reheating. Here, a slab heating temperature may be 1,200° C. or higher. The reason why the heating temperature of the slab is in the above range is for sufficient Ti re-solid solution. When Ti is sufficiently re-solid solubilized, the TiC precipitate is precipitated later.

Meanwhile, a residence time in the slab heating may be 150 minutes or more. When the residence time is too short, re-solid solution of Ti may not sufficiently occur.

Next, the heated slab is hot rolled to manufacture a hot rolled steel sheet. A finish rolling temperature of the hot rolling may be 850 to 1000° C. When the finish rolling temperature is too low, sufficiently rolling ability may not be exhibited, and on the contrary, when the finish rolling temperature is too high, it may be difficult to secure the strength of the steel sheet. Here, the hot rolled plate may have a thickness of 2.5 to 5.5 mm.

Next, a step of winding the hot rolled steel sheet may be included. The step of winding the hot rolled steel sheet may be performed at 450 to 750° C. When the winding temperature is too low, final cold rolling may be difficult due to an increase of the initial strength of the hot rolled material, and on the contrary, when the winding temperature is too high, buckling may occur and strength may be lowered due to the phase transformation in a winding section.

Thereafter, a step of pickling the wound hot rolled steel sheet may be included.

Next, a step of cold rolling the wound hot rolled steel sheet at a reduction rate of 54 to 70% to manufacture a cold rolled steel sheet may be included. When the reduction rate is too low, it may be difficult to secure complete recrystallization in the cold rolling, which causes a decrease in elongation of materials, and cracks in later customer processing. However, when the reduction rate is too high, rolling may not be performed by a motor load in the rolling process.

Next, a step of subjecting the cold rolled steel sheet to an annealing heat treatment at 750 to 880° C. may be included. When the annealing heat treatment temperature is too low, it may be difficult to secure complete recrystallization, which causes a decrease in elongation of materials, and cracks in later customer processing. However, when the annealing heat treatment temperature is too high, it is difficult to secure the strength of the steel sheet.

Hereinafter, the present invention will be described in more detail by the examples. However, the examples are only for illustrating the present invention, and the present invention is not limited thereto.

EXAMPLES

First, a low-carbon steel slab including the alloy components summarized in the following Table 1 was manufactured.

The slab was heated at 1250° C. for 200 minutes, and then hot rolled to a thickness of 3.5 mm, thereby manufacturing a hot rolled plate. A finish rolling temperature (FDT) was 920° C., and winding was performed at 650° C.

TABLE 1 Formula Component Formula 2 Formula system C Si Cu Ni Sb Ti Sn Mo S N 1 (Ti*) 3 Inventive 0.043 0.015 0.28 0.14 0.11 0.072 0.18 0.1 0.005 0.0018 0.50 0.058 19.58 Example 1 Inventive 0.1 0.035 0.26 0.13 0.1 0.07 0.15 0.1 0.007 0.0023 0.50 0.052 18 Example 2 Inventive 0.072 0.035 0.21 0.11 0.12 0.068 0.18 0.1 0.005 0.0021 0.52 0.053 16.3 Example 3 Inventive 0.072 0.032 0.34 0.19 0.1 0.065 0.13 0.1 0.005 0.0022 0.56 0.050 21.76 Example 4 Inventive 0.078 0.044 0.28 0.15 0.052 0.075 0.22 0.1 0.005 0.0032 0.54 0.057 18.784 Example 5 Inventive 0.075 0.035 0.32 0.16 0.145 0.078 0.15 0.1 0.005 0.0038 0.50 0.057 21.99 Example 6 Inventive 0.076 0.032 0.22 0.11 0.11 0.053 0.15 0.1 0.003 0.0018 0.50 0.042 16.22 Example 7 Inventive 0.071 0.022 0.24 0.13 0.11 0.143 0.16 0.1 0.0085 0.0028 0.54 0.121 17.34 Example 8 Inventive 0.073 0.028 0.26 0.14 0.12 0.12 0.075 0.1 0.0065 0.0018 0.54 0.104 17.54 Example 9 Inventive 0.076 0.026 0.28 0.14 0.09 0.09 0.21 0.1 0.0095 0.0023 0.50 0.068 19.5 Example 10 Inventive 0.07 0.02 0.28 0.14 0.1 0.065 0.18 0.06 0.005 0.0024 0.50 0.049 18.96 Example 11 Inventive 0.07 0.012 0.26 0.15 0.1 0.07 0.19 0.14 0.0065 0.0032 0.58 0.049 18.88 Example 12 Inventive 0.07 0.055 0.3 0.15 0.1 0.07 0.2 0.1 0.005 0.0032 0.50 0.052 20.6 Example 13 Comparative 0.035 0.032 0.22 0.15 0.09 0.068 0.15 0.1 0.005 0.0021 0.68 0.053 15.78 Example 1 Comparative 0.12 0.022 0.3 0.15 0.1 0.07 0.2 0.1 0.007 0.0022 0.50 0.052 20.6 Example 2 Comparative 0.07 0.13 0.28 0.15 0.1 0.07 0.18 0.1 0.005 0.0023 0.54 0.055 19.36 Example 3 Comparative 0.072 0.035 0.15 0.14 0.12 0.068 0.18 0.1 0.005 0.0038 0.93 0.047 13.3 Example 4 Comparative 0.072 0.032 0.45 0.15 0.09 0.07 0.15 0.1 0.005 0.0036 0.33 0.050 27.28 Example 5 Comparative 0.078 0.044 0.34 0.1 0.09 0.12 0.09 0.1 0.005 0.0028 0.29 0.103 21.06 Example 6 Comparative 0.075 0.035 0.32 0.22 0.1 0.09 0.13 0.1 0.005 0.0018 0.69 0.076 20.76 Example 7 Comparative 0.076 0.032 0.22 0.16 0.045 0.068 0.22 0.1 0.0085 0.0023 0.73 0.047 15.63 Example 8 Comparative 0.071 0.022 0.24 0.11 0.155 0.07 0.15 0.1 0.0065 0.0021 0.46 0.053 18.21 Example 9 Comparative 0.073 0.028 0.26 0.13 0.12 0.045 0.15 0.1 0.0095 0.0022 0.50 0.023 18.44 Example 10 Comparative 0.076 0.026 0.28 0.14 0.09 0.155 0.16 0.1 0.005 0.0032 0.50 0.137 18.9 Example 11 Comparative 0.075 0.032 0.22 0.14 0.09 0.068 0.06 0.1 0.007 0.0038 0.64 0.044 14.7 Example 12 Comparative 0.076 0.022 0.3 0.15 0.1 0.07 0.23 0.1 0.005 0.0045 0.50 0.047 20.96 Example 13 Comparative 0.075 0.035 0.32 0.16 0.11 0.05 0.15 0.1 0.012 0.002 0.50 0.025 21.22 Example 14 Comparative 0.075 0.035 0.32 0.16 0.11 0.065 0.15 0.1 0.005 0.0055 0.50 0.039 21.22 Example 15 Comparative 0.075 0.035 0.32 0.16 0.11 0.06 0.15 0.1 0.01 0.0049 0.50 0.028 21.22 Example 16 Comparative 0.07 0.025 0.28 0.14 0.09 0.075 0.15 0.04 0.005 0.0021 0.50 0.060 18.18 Example 17 Comparative 0.07 0.03 0.27 0.13 0.095 0.068 0.18 0.2 0.007 0.0022 0.48 0.050 19.75 Example 18

After the low-carbon steel sheet was manufactured, an immersion test was performed by the method described in the standard of ASTM G31. The immersion test was performed by a method of preparing 50 wt % of an aqueous sulfuric acid solution and performing immersion at 70° C. for 6 hours. After the immersion, the steel sheet was washed by the specimen surface washing method of ASTM G1, and a weight loss was measured to measure weight losses per unit time and per unit surface.

In addition, in order to simulate sulfuric acid/hydrochloric acid composite condensation occurring in low-temperature condensation in the Korean-style thermal power plant, a mixed aqueous solution of 28.5 wt % of a sulfuric acid solution and 0.5 wt % of a hydrochloric acid solution was prepared, and a test of immersion at 60° C. for 6 hours was performed. After the immersion, as described above, a weight loss after washing was measured by a specimen surface washing method of ASTM G1, and weight losses per unit time and per unit surface were measured.

The results are shown in the following Table 2. The unit was mg/(cm²*hr).

Meanwhile, in order to reveal the relationship between a corrosion-resistant element and a surface concentrated layer, the hot rolled plates of each of the inventive examples and the comparative examples were immersed in a mixed aqueous solution of 28.5 wt % of a sulfuric acid solution and 0.55 wt % of a hydrochloric acid solution at 60° C. for 24 hours, and then the specimen was measured for an element distribution from the surface to the inside by GDS measurement. The thickness of the concentrated layer measured therefrom and the concentrated amount of the surface concentrating elements were measured and are shown in the following Table 2.

Herein, the concentrated layer refers to a layer in which Mo, Cu, Sb, and Sn starts to be concentrated, and on the other hand, is generally similar to the point at which oxidation starts. Empirically, the thickness of the concentrated layer was measured as the thickness of the layer in which the total amount of Mo, Cu, Sb, and Sn was more than 4 times the total amount of Mo, Cu, Sb, and Sn in the steel sheet. Herein, in the boundary point at which Fe and O met in wt %, it was confirmed that Cu and the like were concentrated at most, and the highest concentrated amount was calculated as the sum of the contents (wt %) of concentrating elements Cu, Sb, and Sn, at a point at which the contents (wt %) of Fe and O were the same. It was confirmed that the concentrated layer formed of Mo, Sb, Sn, and Cu was present at about 10 wt % on the surfaces of the steel material and the corrosion product. It was found that the thickness of the concentrated layer and the concentrated amount determine the corrosion resistance in immersion.

In this regard, FIG. 1 is a graph showing an element concentration degree of a surface portion of a steel sheet, by measuring an element distribution from the surface to the inside by GDS measurement, after the steel sheet of Inventive Example 2 was immersed in a mixed aqueous solution of 28.5 wt % of a sulfuric acid solution and 0.55 wt % of a hydrochloric acid solution for 24 hours. In addition, a boundary point at which Fe and O met corresponds to the red dotted line of FIG. 1 , and the concentrated amount which is the total amount of Mo, Cu, Sb, and Sn in the layer was 13.03 wt %.

In addition, for the manufactured steel sheet, the strength, the hardness, and the crack occurrence thereof were confirmed before acid immersion. The hot rolled materials of the inventive examples and the comparative examples were processed into a tensile specimen conforming to the standards of JIS13B, a tensile test was performed at length in a rolling direction, and the results of measuring HRB surface hardness based on Rockwell hardness are shown in the following Table 2.

In addition, whether cracks occurred in cast iron in the continuous casting process in manufacturing the hot rolled plate, or whether cracks occurred in a hot rolled material edge in the process of hot rolling is also shown in the following Table 2.

TABLE 2 Corrosion Corrosion reduction reduction ratio of ration Thick- sulfuric of ness Cracks acid composite Product of TiC in alone acid of concen- Concen- particle Hard Hot soft mg/ mg/ {circle around (1)} trated trated TiC diam- phase Tensile rolling casting, Component (cm² × hr.) (cm² × hr.) and layer amount density eter ratio strength hardness hot system {circle around (1)} {circle around (2)} {circle around (2)} (nm) (wt %) (/cm³) (nm) (%) (MPa) (HRB) rolling Inventive 25.0 0.19 4.75 15.32 14.2 2.9E+16 3.7 2 670 97 X Example 1 Inventive 36.6 0.24 8.78 14.30 13.0 2.0E+16 3.0 10 730 98.88 X Example 2 Inventive 34.1 0.31 10.57 16.34 11.7 2.2E+16 3.2 3 692 98.365 X Example 3 Inventive 32.0 0.17 5.44 12.26 15.9 1.8E+16 2.8 3 690 98.365 X Example 4 Inventive 32.7 0.27 8.83 14.30 13.6 2.6E+16 3.5 3 694 96.305 X Example 5 Inventive 28.9 0.16 4.62 15.32 16.1 2.7E+16 3.6 6 705 97.335 X Example 6 Inventive 30.5 0.21 6.41 14.30 11.7 1.1E+16 2.1 3 590 87.55 X Example 7 Inventive 30.1 0.44 13.24 14.30 12.5 1.0E+18 9.6 5 760 109.18 X Example 8 Inventive 34.2 0.42 14.36 12.26 12.7 4.0E+17 8.1 5 665 104.03 X Example 9 Inventive 28.9 0.17 4.91 14.30 14.2 5.0E+16 4.6 6 680 99.91 X Example 10 Inventive 31.8 0.44 13.99 18.38 14.1 1.7E+16 2.8 1 664 96.5 X Example 11 Inventive 36.5 0.14 5.11 20.43 13.4 1.7E+16 2.8 12 706 97.3 X Example 12 Inventive 35.3 0.49 17.30 14.30 15.0 1.9E+16 3.0 3 675 93.73 X Example 13 Comparative 38.7 0.36 13.93 9.19 11.3 2.2E+16 3.2 0.5 550 84.46 X Example 1 Comparative 48.9 0.57 27.87 12.26 15.0 2.0E+16 3.0 11 750 94.348 X Example 2 Comparative 32.0 0.88 28.16 14.30 14.8 2.1E+16 2.8 3.2 680 94.1 ◯ Example 3 (Crack occurrence in cast iron) Comparative 72.8 0.86 62.61 7.66 9.4 1.5E+16 2.6 3 655 92.906 X Example 4 Comparative 30.8 0.19 5.85 15.32 20.2 1.8E+16 2.9 2 620 92.7 ◯ Example 5 (Crack occurrence in cast iron) Comparative 31.5 0.22 6.93 15.32 15.4 3.7E+17 7.9 3 650 105.06 ◯ Example 6 (Crack occurrence in cast iron) Comparative 30.9 0.22 6.80 13.28 15.2 8.1E+16 5.4 4 675 96.82 ◯ Example 7 (defect occurrence on surface) Comparative 85.4 1.47 125.54 9.09 11.2 1.5E+16 2.6 2 630 92.7 X Example 8 Comparative 27.0 0.18 4.86 11.23 13.2 2.1E+16 3.1 2 650 92.7 ◯ Example 9 (crack occurrence in hot rolling) Comparative 29.3 0.19 5.57 13.28 13.4 3.8E+15 0.3 3 500 75.19 X Example 10 Comparative 33.8 0.68 22.98 14.30 13.7 2.6E+18 11.2 4 770 111.24 ◯ Example 11 (crack occurrence in cast iron) Comparative 128.3 2.97 381.05 6.95 10.5 1.3E+16 2.3 3 648 92.7 X Example 12 Comparative 36.8 0.16 5.89 14.30 15.3 1.5E+16 2.6 2 645 92.7 ◯ Example 13 (crack occurrence in hot rolling) Comparative 33.5 0.68 22.78 13.28 15.5 4.3E+15 0.5 3 520 78.28 X Example 14 Comparative 32.3 0.59 19.06 12.26 15.5 9.2E+15 1.8 3 560 85.49 X Example 15 Comparative 31.2 0.69 21.53 13.28 15.5 5.1 E+15 0.8 3 530 81.37 X Example 16 Comparative 32.2 0.78 25.12 8.68 13.6 3.2E+16 3.8 0.5 656 91.5 X Example 17 Comparative 57.0 0.11 6.27 15.32 13.6 1.8E+16 2.8 15 780 93.1 ◯ Example 18 (crack occurrence in hot rolling)

In Comparative Example 1 having a low C content, due to the reduced content of the TiC precipitate by the low C content, the tensile strength of the hot rolled material was lower than 580 MPa, and the surface hardness was low, and thus, strength and abrasiveness were not able to be secured. However, when the C content was excessively high as in Comparative Example 2, a phenomenon in which composite corrosion resistance was reduced due to the increased TiC precipitate was observed.

In the present invention, characteristically, the content of Si was greatly lowered, and the reason therefor is that it was confirmed that as the Si content was higher as in Comparative Example 3, red scale excessively occurred on the surface of the hot rolled material, leading to cracks.

In Comparative Example 4 having a low Cu content, in particular, the corrosion resistance to a sulfuric acid alone was reduced, and in Comparative Example 5 having an excessively high Cu content, cracks in cast iron due to the liquefaction of Cu in the continuous casting process was confirmed.

As in Formula 1, the active addition of Ni served to raise the melting point of Cu, and thus, it was confirmed that when a Ni/Cu ratio did not satisfy a certain ratio as in Comparative Example 6, cracks in cast iron occurred.

The most important elements in corrosion resistance are Mo, Cu, Sb, and Sn, and it was found that in Comparative Example 8 having a low Sb content and in Comparative Example 12 having a low Sn content, corrosion resistance to a sulfuric acid alone was greatly deteriorated, and in Comparative Example 17 having a low Mo content, the corrosion resistance to a sulfuric acid alone was similar, but the sulfuric acid/hydrochloric acid composite corrosion resistance was decreased. In Comparative Example 9 having an excessively high Sb content, in Comparative Example 13 having an excessively high Sn content, and in Comparative Example 18 having an excessively high Mo content, it was confirmed that surface defects and cracks of the hot rolled material were caused. In particular, Mo is an element which may form a hard phase such as bainite and martensite, while forming a MoC carbide with C. The hard phase may increase the strength of the hot rolled material as in Inventive Examples 2 and 12 and Comparative Example 2, but when the Mo content is high as in Comparative Example 18, it is known that the hard phase formed at a high fraction deteriorates resistance the corrosion resistance to a sulfuric acid alone.

In the present invention, Ti was actively added for forming precipitates for securing strength and surface hardness, and it was confirmed that when the Ti content was low as in Comparative Example 10, the tensile strength and the surface hardness of the hot rolled material were rapidly lowered. Meanwhile, in Comparative Example 11 having a high Ti content, in particular, in the case of the Ti content of 0.15 wt % or more, nozzle clogging may be caused in the continuous casting process, and extreme nozzle clogging was confirmed even in the real test process of the comparative example.

For forming TiC, not only C and Ti adjustment and temperature adjustment, but also an effective Ti content capable of precipitating carbide is important. The excessive addition of nitrogen and sulfur as in Comparative Examples 14 and 15 lowered the effective Ti content to offset the strength increase effect.

In addition, even in the case in which the composition was within the contents of S and N described in the inventive examples as in Comparative Example 16, when the effective Ti(Ti*) content of Formula 2 was not 0.04 or more, it is difficult to obtain the effects of high strength and high wear resistance.

In order to examine the influence of the manufacturing conditions on the productivity and the strength of the hot rolled material and the cold rolled material, the manufacturing conditions were modified to the component system of Inventive Example 4 to perform manufacture, and the characteristics were evaluated and are shown in Table 3.

TABLE 3 Tensile Tensile strength strength Cold of of Occurrence Reheating reduction Annealing hot cold Cold of Manufacturing temperature FDT CT rate tempenature rolled rolled Recrystallization rolling edge conditions (C.) (° C.) (° C.) (%) (° C.) material material fraction possibility crack Condition 1250 920 650 64 810 690 610 100% ◯ X 1 Condition 1250 800 650 64 810 750 — 100% X — 2 Condition 1250 1050 650 64 810 600 540 100% ◯ X 3 Condition 1250 920 440 64 — 890 — 100% X — 4 Condition 1250 920 755 64 810 610 520 100% ◯ X 5 Condition 1250 920 650 53 810 690 780  70% ◯ X 6 Condition 1250 920 650 72 — 687 — 100% X — 7 Condition 1250 920 650 64 740 690 820  65% ◯ X 8 Condition 1250 920 650 64 890 690 605 100% ◯ X 9 Condition 1180 920 650 64 810 650 570 100% ◯ X 10

In the results of Table 3, in Condition 10 in which the reheating temperature was lower than 1200° C., it was confirmed that the tensile strengths of the hot rolled material and the cold rolled material were decreased even in the case of using the inventive component system, and this is because Ti formed as a precipitate in the slab processing was not sufficiently re-solid solubilized in the reheating process.

In Condition 2 in which a hot finish rolling temperature (FDT) was high, edge cracks occurred in the hot rolling production process, and this was the same in Condition 4 having a low winding temperature (CT). In this regard,

FIG. 2 compares (a) a photograph of a microstructure after Inventive Example 4 was hot rolled under Condition 1, and (b) a photograph of a microstructure after Inventive Example 4 was hot rolled under Condition 4. This shows that a material which had a low CT and a relatively high cooling rate to winding had a high tensile strength of a hot rolled material, and thus, was not able to be rolled to a desired thickness when cold rolling. In particular, when a fine bainite phase is formed in a large amount as in Condition 4, corrosion resistance was poor.

However, under Condition 3 having a high hot finish rolling temperature (FDT) of 1050° C., the tensile strengths of the hot rolled material and the cold rolled material were low, so that a material to be desired was not able to be secured, and this was the same also in Condition 5 having a high winding temperature (CT).

The steel type of the present invention has high contents of C and Ti and is characterized by a high recrystallization temperature after cold rolling, and under Condition 6 having a cold reduction rate of 53%, the recrystallization fraction of the final cold rolled material was about 70%, which did not form complete recrystallization, and in Condition 8 having an annealing temperature was 740° C. which was low also, the recrystallization fraction was 65%, which did not form complete recrystallization. The material which did not undergo complete recrystallization may cause cracks in the customer processing due to a lowered elongation, and thus, in the present invention, when it is used as a cold rolling material, the reduction rate was limited to 54% or more and the annealing temperature was limited to 750° C. or higher.

Further, in Condition 4 and Condition 7 in which the strength of the hot rolled material was high or the cold reduction rate was high, rolling was not performed by a motor loading in the rolling process, and thus, the final product was not able to be obtained.

The present invention is not limited to the implementations and/or the exemplary embodiments, but may be produced in various forms different from each other. A person with ordinary skill in the art to which the present invention pertains will understand that the present invention may be carried out in other specific forms without changing the technical idea or the essential feature of the present invention. Therefore, the implementations and/or the exemplary embodiments described above should be understood to be illustrative in all respects, and not to be restrictive. 

1. A corrosion-resistant steel sheet comprising, by weight: 0.04 to 0.10% of carbon (C), 0.10% or less (excluding 0%) of silicon (Si), 0.20 to 0.35% of copper (Cu), 0.1 to 0.2% of nickel (Ni), 0.05 to 0.15% of antimony (Sb), 0.07 to 0.22% of tin (Sn), 0.05 to 0.15% of titanium (Ti), 0.01% or less (excluding 0%) of sulfur (S), 0.005% or less (excluding 0%) of nitrogen (N), and 0.05 to 0.15% of molybdenum (Mo), with a balance of iron (Fe) and inevitable impurities, the corrosion-resistant steel sheet satisfying the following Formulae 1 and 2: [Ni]/[Cu]≥0.5   [Formula 1] 48×([Ti]/48−[S]/32−[N]/14)≥0.04   [Formula 2] wherein [Ni], [Cu], [Ti], [S], and [N] represent contents (wt %) of Ni, Cu, Ti, S, and N in the steel sheet, respectively.
 2. The corrosion-resistant steel sheet of claim 1, wherein: the corrosion-resistant steel sheet includes a TiC precipitate, and the TiC precipitate and an aggregate formed of the TiC precipitate are included at 10¹⁶ or more per 1 cm³.
 3. The corrosion-resistant steel sheet of claim 2, wherein: the TiC precipitate has a particle diameter of 1 to 10 nm.
 4. The corrosion-resistant steel sheet of claim 1, wherein: the corrosion-resistant steel sheet satisfies the following Formula
 3. 10×[Mo]+12×[Sn]+22×[Sb]+50×[Cu]≥16   [Formula 3] wherein [Mo], [Sn], [Sb], and [Cu] represent contents (wt %) of Mo, Sn, Sb, and Cu in the steel sheet, respectively.
 5. The corrosion-resistant steel sheet of claim 2, wherein: when the steel sheet is immersed in a mixed solution of 28.5 wt % of a sulfuric acid solution and 0.5 wt % of a hydrochloric acid solution at 40 to 80° C., a concentrated layer is produced on a surface of the steel sheet.
 6. The corrosion-resistant steel sheet of claim 2, wherein: when the steel sheet is immersed in 50 wt % of a sulfuric acid solution at 50 to 90° C., a concentrated layer is produced on a surface of the steel sheet.
 7. The corrosion-resistant steel sheet of claim 5, wherein: the concentrated layer includes Mo, Cu, Sb, and Sn.
 8. The corrosion-resistant steel sheet of claim 7, wherein: the concentrated layer has a concentrated amount of 10 wt % or more, wherein the concentrated amount refers to a sum of the contents (wt %) of concentrating elements Mo, Cu, Sb, and Sn, at a boundary point at which Fe and O have the same wt %.
 9. The corrosion-resistant steel sheet of claim 5, wherein: the concentrated layer has a thickness of 10 nm or more.
 10. The corrosion-resistant steel sheet of claim 1, wherein: a recrystallization fraction after subjecting the steel sheet to an annealing heat treatment is 80% or more.
 11. The corrosion-resistant steel sheet of claim 5, wherein: when the steel sheet is immersed in the mixed solution of 28.5 wt % of the sulfuric acid solution and 0.5 wt % of the hydrochloric acid solution at 60° C. for 6 hours, a corrosion weight loss ratio is 0.8 mg/cm²/hr or less.
 12. The corrosion-resistant steel sheet of claim 6, wherein: when the steel sheet is immersed in 50 wt % of the sulfuric acid solution at 70° C. for 6 hours, a corrosion weight loss ratio is 37 mg/cm²/hr or less.
 13. The corrosion-resistant steel sheet of claim 5, wherein: a product of the corrosion weight loss ratio when the steel sheet is immersed in 50 wt % of the sulfuric acid solution at 70° C. for 6 hours and the corrosion weight loss ratio when the steel sheet is immersed in the mixed solution of 28.5 wt % of the sulfuric acid solution and 0.5 wt % of the hydrochloric acid solution at 60° C. for 6 hours is 20 or less.
 14. The corrosion-resistant steel sheet of claim 2, wherein: when the steel sheet is a hot rolled steel sheet, the hot rolled steel sheet has a tensile strength of 580 MPa or more and a surface hardness of 85 or more based on HRB.
 15. The corrosion-resistant steel sheet of claim 2, wherein: when the steel sheet is a cold rolled steel sheet, the cold rolled steel sheet has a tensile strength of 540 MPa or more and a surface hardness of 80 or more based on HRB.
 16. A method of manufacturing a corrosion-resistant steel sheet, the method comprising: preparing a steel slab which includes, by weight: 0.04 to 0.10% of carbon (C), 0.10% or less (excluding 0%) of silicon (Si), 0.20 to 0.35% of copper (Cu), 0.1 to 0.2% of nickel (Ni), 0.05 to 0.15% of antimony (Sb), 0.07 to 0.22% of tin (Sn), 0.05 to 0.15% of titanium (Ti), 0.01% or less (excluding 0%) of sulfur (S), 0.005% or less (excluding 0%) of nitrogen (N), and 0.05 to 0.15% of molybdenum (Mo), with a balance of iron (Fe) and inevitable impurities, and satisfies the following Formulae 1 and 2; heating the slab at 1,200° C. or higher; and hot rolling the heated slab at a finish rolling temperature of 850 to 1000° C. to manufacture a hot rolled steel sheet: [Ni]/[Cu]≥0.5   [Formula 1] 48×([Ti]/48−[S]/32−[N]/14)≥0.04   [Formula 2] wherein [Ni], [Cu], [Ti], [S], and [N] represent contents (wt %) of Ni, Cu, Ti, S, and N in the steel sheet, respectively
 17. The method of manufacturing a corrosion-resistant steel sheet of claim 16, further comprising: after the manufacturing of a hot rolled steel sheet, winding the hot rolled steel sheet at 450 to 750 ° C.; cold rolling the wound hot rolled steel sheet at a reduction ratio of 54 to 70% to manufacture a cold rolled steel sheet; and subjecting the cold rolled steel sheet to an annealing heat treatment at 750 to 880 ° C.
 18. The method of manufacturing a corrosion-resistant steel sheet of claim 16, wherein: in the heating of a slab at 1,200° C. or higher, a residence time is 150 minutes or more.
 19. The corrosion-resistant steel sheet of claim 6, wherein: the concentrated layer has a thickness of 10 nm or more.
 20. The corrosion-resistant steel sheet of claim 6, wherein: a product of the corrosion weight loss ratio when the steel sheet is immersed in 50 wt % of the sulfuric acid solution at 70° C. for 6 hours and the corrosion weight loss ratio when the steel sheet is immersed in the mixed solution of 28.5 wt % of the sulfuric acid solution and 0.5 wt % of the hydrochloric acid solution at 60° C. for 6 hours is 20 or less. 